The present invention relates to a method of forming a laser butt weld and, more particularly, to a method of progressively forming a laser butt weld between opposed, longitudinally extending edges to be welded, such as the opposite side edges of a sheet of material which has been formed into a tubular-shaped workpiece for making a can body or the like.
Progressively forming a laser butt weld along opposed edges of two members has recently been made possible by innovations such as that suggested by Saurin et al in U.S. Pat. No. 4,152,573 wherein the abutting edges to be welded are clamped relative to one another and then moved relative to a laser beam, and also that disclosed by Nilsen in U.S. Pat. No. 4,354,090 wherein a Z-bar guide apparatus is used for guiding the opposed edges to be welded into substantially abutting edge-to-edge relationship with one another for laser welding. With both of these known laser welding techniques, the edges to be welded are typically sheared edges of a material such as sheet metal, so that the edges extend essentially perpendicular to the plane of the material adjacent the edge. The adjacent edges are positioned in abutting relationship along essentially the entire thickness of the joint to form a closed butt joint which is welded by directing a laser beam at the weld joint from one side thereof.
It has been discovered that in forming such a butt joint between two members lying approximately in the same plane, there may be problems with inconsistent or irregular weld formation along the length of the members being welded, especially where welding is to be accomplished at a relatively high speed and with minimum heat input so as to reduce heat input related metallurgical problems and/or fluid flow problems known as slubbing. Fluctuations in the transfer efficiency of the laser energy to the edges to be welded along the length of the weld joint is one reason for this problem. More specifically, it has been found that the presence of burrs on one or more of the sheared edges to be welded contributes to this inconsistency. That is, when a material such as sheet metal, for example, is sheared, typically a burr is formed at the sheared edge, which extends outwardly from the plane of the sheet material in the direction of shearing. In the case where a burr extends outwardly from the side of a butt joint to be laser welded, and the laser beam approaches the joint on the side of the burr, at the instance of impingement the burr tends to scatter the laser energy and reduce the efficiency with which the laser energy can be transmitted to the edges for welding. Moreover, because the burrs are not of great mass as compared with the edges to be welded, they are vaporized in a short period of time after being contacted by the laser beam. This vaporization can further obstruct the transmission of laser energy to the edges to be welded and contribute to the inititation of a weld plasma in the area of the burr. The presence of the plasma may also further reduce the effectiveness of energy transmission to the edges for welding. Thus, the intermittent presence and often irregular configuration of burrs along the edges to be laser butt welded can result in incomplete weld penetration and weld formation, especially where a minimum heat input for welding abutting edges is employed. On the other hand, if the laser beam energy is increased so as to be sufficient to effect a weld even in the presence of burrs, in those areas having little or no burrs, an excess of heat input will exist for the formation of a sound weld. This will increase the size of the weld and weld heat affected zone, and also increase heat input related metallurgical problems and possibly slubbing. Thus, the presence of burrs can result in weld defects and also a reduction in the efficiency with which edges can be continuously laser butt welded.
The problem of course can be avoided by grinding sheared edges to be welded prior to welding to remove the burrs. However, this is undesirable because it adds an additional manufacturing step, and the consequent time and expense associated therewith, to the production process. Another possible solution is to ensure that any burrs on abutting edges extend outwardly from the weld joint on the side of the weld joint away from that upon which the laser beam is to initially impinge. Unfortunately, for many applications, this is not a practical solution since the burrs formed on opposite sides of a sheared piece of material will extend in opposite directions due to the construction and operation of the existing slitter or shearing apparatus. Forming a sheared piece of material into a tubular-shape for butt welding will then produce a weld joint with a burr protruding from both sides thereof.
An object of the present invention is to provide a method of forming a laser butt weld wherein the adverse effect of the presence of a burr on the side of the weld joint initially impinged by the laser beam, is avoided, so that more efficient, uniform welding of abutting edges can be accomplished at high speed, while minimizing heat input and heat input related problems such as slubbing.
A further object of the invention is to provide a method of progressively forming a laser butt weld between opposed abutting edges of two members, which method permits an increased welding speed with a given heat input and thereby a more efficient transfer of laser energy to the workpieces, so that a very narrow, uniform weld can be formed with a relatively small heat affected zone.
An additional object of the invention is to provide a method of the aforementioned type which permits keyhole welding of abutting edges to be accomplished at high speed and high efficiency to produce a weld having a relatively uniform cross section through the entire thickness of the members being joined.
These and other objects of the invention are attained by the method of the invention which comprises the steps of providing two members to be butt welded, positioning respective edges of the members in abutting contact with the two members being canted slightly with respect to one another so that the butt joint formed by the edges is open toward a first side of the members with the edges contacting one another toward a second side of said members opposite said first side, and effecting a butt weld at the butt joint by directing a high energy density laser beam at the butt joint while urging the edges of the two members in the direction of the butt joint, the laser beam being directed at the butt joint from the first side of the members so that at least a portion of the laser beam extends into the open side of the butt joint to the edges of the members.
According to a disclosed, preferred embodiment of the method, the butt weld between the two members is continuously formed along the length of the edges of the members by moving at least one of the laser beam and the members relative to the other in a direction along the butt joint to progressively weld the edges. In this embodiment, the two members are the opposite sides of a single piece of sheet material, and the step of providing the two members to be butt welded includes forming the piece of sheet material into a tubular-shaped workpiece having opposed longitudinally extending edges to be butt welded. The step of positioning the edges includes moving the tubular-shaped workpiece relative to a Z-bar guide apparatus with the edges being guided by the Z-bar guide apparatus into the abutting contact as described above. The method further includes the step of supporting the tubular-shaped workpiece by support means as it is moved relative to the Z-bar guide apparatus, the location of the support means being adjusted with respect to the Z-bar guide apparatus to effect the slight canting of the opposed sides of the workpiece with respect to one another. The tubular-shaped workpiece disclosed is generally cylindrical in cross section. It is supported by the support means relative to the Z-bar guide apparatus such that it has a slightly pear-shaped cross section with the edges to be welded located at the top thereof to thereby cant the opposed sides of the workpiece with respect to one another and open the joint on the outer side. This causes any burrs on that side of the joint to be pulled outwardly from the center of the joint so as to offer reduced or no interference to the incoming laser beam as compared to the conventional methods. Also, at least a portion of the approaching laser beam is free to pass directly into the open side of the butt joint to the edges for heating and melting the material thereof.
In the disclosed embodiments, the edges of the members to be welded are essentially straight sheared edges extending approximately perpendicular to the plane of the adjacent member. Burrs are present on the edges from the shearing operation. Thus, when the edges are positioned according to the invention, they make, as seen in a sectional view taken transverse to the edges, essentially point contact with one another at the second side of the members with the edges tapering away from each other in the direction toward the first side of the members.
Further, according to the invention, the high energy density laser beam preferably penetrates completely through the butt joint to effect keyhole welding of the edges. When this occurs, an additional step of the method includes providing a laser beam dump on the second side of the members to absorb the laser beam passing through the butt joint during the keyhole welding. This prevents damage to the inside of a tubular workpiece such as a can body or the like, for example, as well as collecting any debris expelled from the weld keyhole. The safety of the welding operators is also enhanced with the use of such a beam dump which prevents unwanted reflectance of the laser energy. Alternatively, the method may include providing a means to accurately reflect the laser beam which has passed through the butt joint back toward the joint to slow the cooling rate thereof.
When the laser beam penetrates completely through the butt joint to form a keyhole, molten edge material surrounds the keyhole. By urging the two members being welded in the direction of the butt joint during welding according to the method, the molten edges of the members are mashed together immediately behind the keyhole as the keyhole and laser beam move relative to the members in a direction along the butt joint.
A second embodiment of the invention comprises clamping the members to be welded in a welding jig with clamping means to position the respective edges in the described abutting contact.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, two embodiments in accordance with the present invention.